I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for processing a received modulated signal using an equalizer and a rake receiver for improved performance.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system that supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD. FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM.” Another specific CDMA system is disclosed in U.S. patent application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, now U.S. Pat. No. 6,574,211, issued Jun. 3, 2003, (hereinafter, the HDR system). These patents are assigned to the assignee of the present invention and incorporated herein by reference.
A CDMA system is typically designed to conform to one or more standards. Such standards include the “TIA/EIA/IS-95 Remote Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), and the “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the CDMA-2000 standard). New CDMA standards are continually proposed and adopted for use. These CDMA standards are incorporated herein by reference.
A CDMA system typically employs a rake receiver to process a modulated signal that has been transmitted on the forward or reverse link. The rake receiver typically includes a searcher element and a number of finger processors. The searcher element searches for strong instances of the received signal (or multipaths). The finger processors are assigned to process the strongest multipaths to generate demodulated symbols for those multipaths. The rake receiver then combines the demodulated symbols from all assigned finger processors to generate recovered symbols that are estimates of the transmitted data. The rake receiver efficiently combines energy received via multiple signal paths.
The rake receiver provides an acceptable level of performance for CDMA systems operated at low signal-to-noise ratio (S/N). For CDMA systems designed to transmit data at high data rates, such as the HDR system, higher S/N is required. To achieve the higher S/N, the components that make up the noise term N need to be reduced. The noise term includes thermal noise (No), interference (Io) due to transmissions by other transmitting sources and transmissions for other users, and inter-symbol interference (ISI) that can come from multipath and distortion in the transmission channel. For CDMA systems designed to operate at low S/N, the ISI component is typically negligible compared to other noise components. However, for CDMA systems designed to operate at higher S/N, the other noise components (e.g., interference from other transmission sources) are typically Freduced, and ISI becomes a non-negligible noise component that may have a large impact on the overall S/N.
As noted above, the rake receiver provides acceptable performance when the S/N of the received signal is low. The rake receiver can be used to combine energy from various multipaths, but generally does not remove the effects of ISI (e.g., from multipath and channel distortion). Thus, the rake receiver may not be capable of achieving the higher S/N required by CDMA systems operating at higher data rates.
As can be seen, techniques that can be used to process a received modulated signal to achieve higher S/N needed to support higher data rates are highly desirable.